1. Field of the Invention
The present invention relates to a method for manufacturing an airtight vessel. The present invention especially relates to a method for evacuating gases discharged while sealing an evacuation tube in the method for manufacturing the airtight vessel to be used for an image-forming apparatus.
2. Description of Related Art
Electron emission elements known in the art have been mainly categorized into a thermionic-cathode and a cold-cathode. The cold-cathode include a field emission type (referred to FE-type hereinafter), metal/insulation layer/metal type (referred to MIM-type hereinafter) and surface conduction type electron emission elements.
Examples of the FE-type electron emission elements are disclosed by W. P. Dyke and W. W. Dolan in “Field Emission”, Advance in Electron Physics, 8, 89 (1956) or by C. A. Spindt in “Physical Properties of Thin-film Field Emission Cathodes with Molybdenum Cones”, J. Appl. Phys., 47, 5248 (1976).
Examples of the MIM-type electron emission elements are disclosed by C. A. Mead in “Operation of Tunnel-Emission Devices”, J. Appl. Phys., 32, 646 (1961).
Examples of the surface conduction type electron emission elements are disclosed by M. I. Elinson in Radio Eng. Electron Phys., 10, 1290, (1965).
The surface conduction type electron emission element makes use of a phenomenon in which electrons are emitted by allowing an electric current to flow through a small area thin film formed on a substrate parallel to the film surface. Elements using a SnO2 thin film (Elinon et. al.), using an Au thin film (G. Dittmer, “Thin Solid Films”, 9, 317, 1972), using an In2O3/SnO2 thin film (M. Hartwell and C. G. Fonstad, IEEE Trans., ED Conf., 519, 1975) and using a carbon thin film (Hisashi Araki et, al., Shinku (Vacuum), 26, 1, p22, 1983) are reported as this surface conduction type electron emission element.
Flat panel display devices that make fluorescent films to undergo light emission by an electron beam projected out of these cold-cathode electron emission elements have been developed.
The foregoing display device requires an ultra-high vacuum in order to steadily operate the cold-cathode electron emission element for a long period of time. The display device is composed of an airtight vessel in which a frame is inserted between a substrate having a plurality of electron emission elements and a confronting substrate having fluorescent films, and the vessel is sealed by a method to be described hereinafter.
In order to manufacture an airtight vessel inside of which maintains a high vacuum as described above, the inside of the vessel is at first evacuated with a vacuum pump through an evacuation tube connected to the vessel in the conventional art. Inside of the vessel is then sufficiently degassed by a baking step by keeping the vessel at a high temperature of 300 to 350° C. for several hours. After cooling the vessel to room temperature, an evaporable getter placed in the vessel and mainly composed of Ba is heated with microwave or by flowing an electric current to form a getter film by allowing the Ba-containing getter to evaporate (referred to getter flash hereinafter). The airtight vessel is cut off from the evacuation tube after sealing a part of the evacuation tube connected to the vacuum pump by heat-fusion. Vacuum in the airtight vessel is maintained with the getter film.
The method for maintaining an ultra-high vacuum in the vessel is disclosed, for example, in Japanese Patent Laid-Open No. 7-302545. In this method, gases adsorbed at interior surfaces of the display device can be readily discharged by repeating the step for introducing and holding the gas into the display device while baking the inside of the display device after evacuation followed by repeating the evacuation steps of the display device several times, thereby enabling to reduce the amount of adsorbed gases in the display device to maintain an ultra-high vacuum in the display device.
Japanese Patent Laid-Open No. 7-296748 also discloses the steps of readily degassing the inside of the vessel by baking the vessel, activating the getter disposed in the evacuation tube and forming an airtight vessel by sealing the evacuation tube. Either a evaporable getter or nonevaporable getter may be used as a getter.
Japanese Patent Laid-Open No. 7-296731 discloses a different sealing method. According to the patent publication above, an airtight vessel is obtained by sealing the evacuation tube after heating the tube at a temperature higher than the temperature for heating the vessel during the baking step of the vessel.
However, there have been problems as described below for maintaining a vacuum with the foregoing getter.
The pressure P in the airtight vessel is represented by an equation “P=Q/S”, wherein Q is the amount of discharged gas from the surface of the vessel and S is an effective evacuation rate. The effective evacuation rate is determined by the configuration of the airtight vessel, position of the getter and evacuation rate of the getter. In other words, when the configuration of the airtight vessel, the position of the getter and the evacuation rate of the getter are fixed, the amount of discharged gas Q in the airtight vessel should be diminished in order to reduce the gas pressure in the airtight vessel as low as possible. A sufficient degassing treatment such as the baking treatment is necessary for this purpose prior to the sealing step. However, gases are discharged again during the sealing step.
The amount of the discharged gas is about 5×10−7 to 1×10−5 Pa·m3, and the main component of the discharged gas is water. In the gases generated by sealing, it is conjectured that substances incorporated into the glass constituting the evacuation tube are discharged from the glass by being heated above its softening point during sealing of the evacuation tube. Most of the gases discharged by sealing are trapped in the airtight vessel, again contaminating the room of the airtight vessel that has been once cleaned by the baking step.
The gases generated form the evacuation tube during sealing are removed by the getter disposed in the evacuation tube according to Japanese Patent Laid-Open No. 7-296748 cited above. However, such a construction makes evacuation time of the vessel too long or the diameter of the evacuation tube itself should be large owing to the getter in the evacuation tube.
When the diameter of the evacuation tube is large, sealing it becomes difficult thus failing to maintain the performance of the airtight vessel.
Impurities such as water, oxygen and CO should be reduced to as small as possible in the flat type image display device making use of the field emission type electron emission element and surface conduction type electron emission element in order to stabilize electron emission characteristics of electron sources. Accordingly, the evacuation method as hitherto described had a drawback that the electron emission characteristics of the electron sources are not stabilized owing to impurities discharged in the sealing step, thereby decreasing the life span of the device.